But the promise of a relatively high-margin boost to output from 2015, making BP's long-term production growth forecast of 1% to 2% look more credible, should underpin BP.

The deal catalyst is Devon's decision to refocus on onshore U.S. natural-gas reserves. That makes the Oklahoma company a seller of deep-water assets.

The deal gives BP a foothold in Brazil, home to the biggest offshore oil discoveries in the Americas since the 1970s but now off-limits to foreign companies. Among the Brazilian assets BP has acquired are blocks in the Campos basin, where three discoveries suggest oil fields of at least 500 million barrels each. The nearby Santos basin, operated by Brazil's Petrobras, may hold eight billion barrels of oil.

BP also consolidates its U.S. Gulf of Mexico position, where it is the biggest producer, and Azerbaijan, adding future production in areas it knows well. Part of the transaction involves the $500 million sale of undeveloped oil-sand leases in Canada to Devon.

But BP seems to be paying a full price. Devon had penciled in $4.5 billion to $7.5 billion from asset sales and has already raised $1.3 billion.

BP gains only 40,000 barrels of extra daily production compared with its total output 3.8 million barrels. The question is whether BP gets the longer term boost it expects. That will be a real test of the firm's competitive edge in deepwater exploration and production.

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